ARD or lift inverter: which backup does your lift need?
Both sit in the machine room and both run off batteries. They are answering different questions, and the sizing arithmetic is not the same one.
The distinction is scope, and it decides almost everything downstream. An automatic rescue device gets the car to a landing and opens the doors. A lift inverter keeps the lift in normal service — full speed, full travel, calls answered — for as long as the bank lasts. An ARD prevents entrapment. An inverter prevents interruption.
A residential building with a working generator usually needs the first. A hospital, a hotel, or any building where the lift stopping is itself the problem usually wants the second. Under IS 17900 the ARD is not optional on a new installation, so for most buildings the real question is whether an inverter is added on top of it.
Why an ARD can be far smaller than the machine
This is the part that surprises people looking at a 15 kW machine and a rescue device the size of a filing cabinet. The ARD does not lift the load. On detecting a supply failure it works out which direction requires the least current and drives that way.
In a traction lift the counterweight is normally matched to the car plus about half of rated load. So the car and the counterweight are rarely in balance, and whichever side is heavier will move if you let it. The rescue device is largely controlling a descent rather than powering a lift, which is why it can be rated well below the machine and why rescue speed is a fraction of contract speed. Slow is not a limitation here; it is the mechanism working correctly.
If the car happens to be sitting at a landing when the supply fails, there is nothing to rescue and the device simply opens the doors.
The failure that catches bench-tested installations
Regeneration. When a loaded car descends — or an empty car rises, pulled by the counterweight — the machine is being driven rather than driving, and it acts as a generator. That energy comes back down the motor cables and the drive rectifies it onto its DC bus.
On mains that is a solved problem: the energy goes to a braking resistor, or a regenerative front end pushes it back into the supply. On battery, during a rescue, it has to go somewhere. If nothing can absorb or dump it, the DC bus voltage climbs and the drive trips on overvoltage — halfway through the rescue, with the car between floors, which is precisely the situation the device was fitted to prevent.
A rescue device tested on an empty car going the easy direction will pass. The same device with a full car and the other direction will not. Ask how the arrangement handles regenerated energy before you buy it, and ask for the rescue to be demonstrated loaded.
The brake is not the drive
Whatever the backup arrangement does, it never substitutes for the mechanical brake. A gravity-loaded machine requires independent mechanical braking, and no drive, inverter or rescue device changes that. If a proposal has the electronics holding the load, it is the wrong proposal.
Single-phase output: cheap, and sometimes false economy
Rescue devices come with single-phase and three-phase outputs, and the single-phase units are common on smaller machines. They work, but the derating is real and it is often left out of the comparison.
Feeding a three-phase drive from a single-phase source roughly doubles the RMS input current for the same delivered load. The input diodes carry it, the DC bus ripple rises, the bus capacitor runs hotter, and input harmonics and power factor both get worse. Manufacturers derate the drive substantially for single-phase input for exactly these reasons — and terminal blocks rated for continuous RMS are not necessarily rated for the peak current that a single-phase rectifier draws in pulses.
None of that stops it working on the day it is commissioned. It shows up as capacitor life, which is to say it shows up in year four.
| ARD | Lift inverter | |
|---|---|---|
| Purpose | Move to nearest landing, open doors, stop | Keep the lift in normal service |
| Travel | One landing | Full travel, calls answered |
| Speed | Reduced rescue speed | Contract speed |
| Rating vs machine | Well below — uses the car/counterweight imbalance | Sized against the machine and its starting duty |
| Typical building | Residential with a generator | Hospital, hotel, no DG |
| Under IS 17900 | Required on new installations | Specified on top, where continuity matters |
Sizing an inverter, as opposed to a rescue device
Once you want normal service the imbalance trick stops helping, because the lift has to answer calls in both directions with any load. The inverter is now sized against the machine, and the number that governs is starting duty rather than running kW. A lift motor draws well above its running current on every start, and a lift starts constantly.
Two other figures decide the battery rather than the electronics. How many trips per hour the building actually makes, which sets the discharge profile, and how quickly the bank must be ready again after an outage — a building with two outages in an afternoon needs recharge capacity, not just autonomy. Sizing for hours of backup and ignoring recharge is the standard mistake.
What to send with an enquiry
- Machine rating in kW, supply voltage and whether the machine is geared or gearless.
- Controller make and model, and the drive type. The interface differs between VVVF controllers and it is the thing that decides whether a retrofit is straightforward.
- Whether the existing drive is regenerative, or has a braking resistor fitted. This determines how the regenerated energy is handled during a rescue.
- Number of floors, contract speed and rated load.
- Whether a generator is present, and how long its changeover takes — that gap is the ARD case even in a building that thinks it is covered.
- Machine room space, ventilation and summer temperature, because that is what battery life is actually governed by.
On anything we have not interfaced with before, the machine room gets surveyed before we quote. It is cheaper for everyone than discovering the controller signalling on commissioning day.
Questions we get asked
Anything not covered here, call us — most of it is faster to answer on the phone.
01What is the difference between an ARD and a lift inverter?
Scope. An ARD moves the car to the nearest landing, opens the doors and stops — it prevents entrapment. A lift inverter keeps the lift in normal service at normal speed for as long as the bank lasts — it prevents interruption. They are often specified together, the ARD because the code requires it and the inverter because the building cannot tolerate the lift stopping.
02Why is a rescue device rated so much lower than the lift motor?
Because it is not lifting the load. The counterweight is matched to the car plus roughly half rated load, so the two sides are rarely balanced, and the device drives in whichever direction needs least current — usually controlling a descent rather than powering a lift. That is also why rescue speed is a fraction of contract speed.
03Why does the drive trip during a rescue?
Most often regeneration. A descending loaded car drives the machine as a generator and that energy is rectified onto the drive DC bus. On mains it goes to a braking resistor or back to the supply; on battery it has to be absorbed or dumped, and if it is not, the bus rises and the drive trips on overvoltage mid-travel. Ask for the rescue to be demonstrated with a loaded car, not an empty one.
04Is a single-phase ARD good enough?
On smaller machines it is common and it works, but the derating is real. Single-phase input to a three-phase drive roughly doubles the RMS input current for the same load, raises DC bus ripple and runs the bus capacitor hotter, which is why manufacturers derate substantially. It rarely fails at commissioning; it shows up as capacitor life a few years in.
05Can an ARD or inverter be retrofitted to an existing lift?
In most cases yes, and it does not normally require replacing the drive. The constraint is the controller interface rather than the rating — VVVF controllers differ in how they signal a supply failure and how they expect the rescue sequence to be initiated, so the machine room is surveyed first.
06Does a lift on a generator still need backup?
Usually, because of the changeover gap. A generator takes time to start, accept load and stabilise, and during that window the car is stationary between floors with passengers in it. It also covers the case every building eventually meets — the DG that does not start. Lift starting current is also one of the loads most often underestimated when a DG is sized.
07How long should a lift inverter run for?
Long enough for the building rather than for a round number. What matters more than raw autonomy is the trip rate during the outage, which sets the discharge profile, and how fast the bank recharges afterwards — a building that loses supply twice in an afternoon needs recharge capacity as much as it needs hours.
08Does the backup replace the mechanical brake?
No, and any proposal that implies it should be rejected. A gravity-loaded machine requires independent mechanical braking. The electronics control motion; the brake holds the load.
09What decides battery life in a machine room?
Temperature, mostly. A bank in an unventilated machine room under a terrace roof ages far faster than the datasheet suggests, and it ages quietly — the charger reads normal on float right up until the discharge it cannot support. Ventilation and an annual discharge test matter more than the brand on the cell.
10Which is required by IS 17900?
The ARD. Since 22 December 2025 it is mandatory for new installations, safety components and major modernisations — a point where the Indian code goes beyond EN 81-20, which only recommends it. An inverter is a continuity choice on top of that requirement, not an alternative to it.